Problem has been solved far more easily than I expected. Yday I decided to isolate the thin brown/yellow wire and checked the battery drain again. This time it went to zero. That narrowed down the possibilities considerably. I then replaced the ignition bulb in the cluster just to make sure that it is a working bulb. Reconnected the br/yellow wire to the original connection in the car's harness (see my post above) - the one which when connected the first time to the new alternator, produced no charge when starting the engine! I was expecting the same result this time round but amazingly, the ignition light came on when I first turned the key and the voltmeter started climbing when I started the engine! I am a bit baffled how it has sorted itself out but am very happy with the result and definitely won't complain! I left the battery connections on and this morning the battery read a healthy 12.6v